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Transcript
6-1 Oceanography Composition of the Oceans: Study of the Earth’s oceans. By weight, seawater contains ~3.5% materials other than water. Draws upon a variety of disciplines. Most of these are salts, predominantly sodiumchloride (table salt). 70% of the Earth’s surface is covered by oceans. Many of these minerals brought to the oceans from land sediments. Clearly the oceans play an important role in the Earth system as a whole. Volcanoes also release gases into the oceans. While the average salinity of the oceans is 35 parts per thousand (35 ‰) it varies significantly. Salinity is relatively high in regions with significant evaporation. Where would we expect this to occur? What would we expect in regions of heavy rainfall? What about regions of large inflow of water from the continents? Layers in the Oceans The ocean can be vertically split into three major units: Near the surface is a region heated by sunlight. Turbulence in the water tends to mix this layer well leading to a nearly constant temperature of around 21–26°C. Below this layer is a transition zone where temperatures fall rapidly. The final layer comprising about 80% of the ocean is a deep layer at ~4°C. 6-2 Topography of the Ocean Floor While some research had been done prior to WWII most of the sea floor remained a mystery prior to that time. They found it is not flat as they had supposed. They found a long mountain range running down the middle of the Atlantic ocean. The mid-Atlantic ridge. Geologists assumed it to be an essentially flat featureless place. However, WWII led to a new urgency to develop technology to detect underwater objects. It is now known that this ridge is much more extensive and winds its way over 40,000 miles through all the Earth’s oceans. Now called the mid-Oceanic ridge. New technology, some developed during the war, allowed geologists to examine the sea floor. As we shall see later, this discovery helped lead to the revival of the idea of continental drift and plate tectonics. Active Plate Margins The mid-oceanic ridge is thought to be a region of current formation of new crust. What I have just described is what is seen at passive plate margins where there is no motion between the continent and oceanic plates. Plates are forced apart in these regions. For example, the coasts along the Atlantic ocean. Very distinctive double ridged structure to the midoceanic ridge: However, the topography is quite different where the plates are converging. For example around much of the Pacific. These are active plate margins. Not like features seen in mountain ranges on land. 6-3 At such boundaries the continental shelf is greatly reduced to practically nonexistent. Instead such margins are marked by deep-ocean trenches. These are the deepest portions of the ocean and can extend to depths of over 11 km. Associated with substantial geologic activity (earthquakes, volcanoes, and mountain building). Other Features of the Sea Floor In addition to the features just mentioned some additional features are often seen. Underwater volcanism is seen at a number of locations. Some of these volcanoes become big enough to poke above sea level to become volcanic islands. Examples? However, sometimes they don’t break the surface. The ocean floor is dotted by a number of these volcanoes which are called sea mounts. Oftentimes chains of these volcanoes are seen. Coral Reefs and Atolls Coral reefs are constructed primarily from the skeletal remains of corals and algae. Often these features will form rings surrounding a central lagoon called atolls. Associated with volcanism at hot spots. Creatures which create coral reefs only exist near the surface and in warm waters. Once formed, a volcanic island will eventually be eroded to near sea level. These may then become submerged as they move to deeper locations. These form distinctive flat-topped features called guyots. Thus most coral reefs are found in warm waters, particularly in the Indian and Pacific oceans. Atolls thought to be formed around volcanic islands. As the island submerges, coral continues to build upon the preexisting coral. 6-4 Composition of the Sea Floor Origin of Sea Floor Sediments Oceanic crust is continually being created at the mid- The sea floor sediments are derived from a variety of sources but most ultimately derive from land. oceanic ridge. Mid-oceanic volcanism results mostly in the rock basalt and its intrusive cousin gabbro. Terrigenous Sediments These sediments are derived directly from land. Thus the oceanic crust consists almost entirely of these rocks. They consist mostly of small mud-sized solid particles created from the weathering of rocks. The crust is on average about 7km thick with a 2km thick layer of basalt atop ~5km of gabbro. Carried to the ocean by rivers or other media. These rocks are overlain by a thin (~200m) layer of sediments. They eventually settle out onto the sea floor. Biogenous Sediments Hydrogenous Sediments Some sediments are created by marine life. Some sediments precipitate directly from the water Creatures take material out of solution in the water and use it for their own purposes creating shells and skeletons and other hard parts. Examples include some calcium carbonates and silica (SiO2). When these creatures die they rain down on the sea floor to become deposits. Most of this material and the material used by sea creatures was derived from land. Thus, ultimately, these sediments have their source on land as well. 6-5 Ocean Currents Surface currents on the oceans are driven predominantly by winds. Deep Ocean Currents In addition to the surface currents, the deep water undergoes circulation as well. Friction between the air and water transfers energy from the air to the water. When air blows over the water steadily along a given direction it sets ocean currents in motion. Winds are also responsible for creating most of the ocean waves we see. If winds really do drive ocean currents, what pattern in the ocean currents would we expect to see? These currents are driven by density differences in the water. Increased salinity raises the density of water. Decreased temperature has the same effect. Salty, cold water will be relatively dense and may sink towards the ocean bottom. As water sinks it pulls more water after it. Water at depth then flows back to the origin of the subsiding water setting up a circulation pattern. When ocean currents change, substantial changes in the climate can result. For example, at times water from near the equator flows towards high northern latitudes. Ocean Currents and Climate As it flows through the region of the subtropical high it suffers evaporation increasing salinity. Ocean currents (both surface and deep water) can have a great influence on climate. However it is still warm so it doesn’t sink. The air circulation pattern transports the most energy from the equator towards the poles. When it reaches the colder more northerly regions it cools down and eventually sinks. However, ocean currents play an important role as well (transporting ~25% of the total). Water then returns at depth. 6-6 Shore and Coastal Processes Coastal regions are amongst the most popular. Water Waves Most of the energy which is available to work on shores is brought by water waves. People flock to beaches for relaxation. Beachfront property is highly sought after. However, coastal regions are quite fragile. Most water waves are generated by winds blowing over the ocean surface. Size of waves will be greatest where winds blow from a constant direction over long distances. Our actions can have unintended consequences. Understanding the processes involved can help us avoid costly mistakes. Also of geologic interest. Characteristics of Waves A water wave can be characterized by its height, wavelength, and period. The height of a wave is the vertical distance between adjacent wave crests and troughs. Given this, in which ocean would you expect to see the largest waves? In which hemisphere? The period of a wave is the time it takes a wave to pass a stationary point. What are typical periods for a wave? The velocity of a wave is related to the wavelength and period by: A wave’s energy is determined by its height. Typical wave heights in the open ocean are a few meters. The wavelength of a wave is the distance (crest to crest or trough to trough) between two waves. Typical wavelengths are 40 to 400 meters. velocity = wavelength Period What are typical velocities of water waves? 6-7 Wave Motion at the Shore Waves may travel for great distances. Eventually the water at the top of the wave overruns the water lower down. The wave “breaks.” However, when they approach a shore the ocean depth becomes shallower and shallower. The water now moves towards the shore as surf. Eventually the wave motion begins to be restricted by the ocean bottom: It is in the surf region where the energy of the wave is expended. friction starts to slow the wave down. Energy is used to erode and transport material. The wavelength decreases and the height starts to increase. Coastal Erosion If the sea floor slopes gently waves will break farther offshore. Waves battering the shore can break up and loosen coastal rocks. Much of its energy will then be dissipated away from the shore. The amount of erosion which takes place depends on what? Where the sea floor is steeply sloped (e.g. at active plate margins) waves may hit the shore with full force. Wave action will tend to straighten out coast lines. To see why consider what happens as a wave approaches a jagged shore... 6-8 Coastal Transport and Deposition Where does the sand come from? Much like rivers or wind, wave action can result in transport of material. Material is often transported by the longshore current. Often times the material being transported is sand sized particles forming a beach. Beaches form a relatively narrow zone where wave action takes place. Formed of material being transported. If not disturbed by outside forces, a beach will eventually reach an equilibrium. Deposition and Depositional Landforms Material from rivers or other sources is brought to the system. As with rivers or wind, coastal processes will deposit material when it is no longer capable of transporting it. Material is then transported away by coastal action This can occur for a variety of reasons: When these are equal the beach has reached an equilibrium and will remain stable. 1. Wind velocity and wave action may vary seasonally. Disrupt the system though and the beach may quickly erode away—or grow. Often, wave action is more vigorous in winter with increased storm activity, less so in summer: 6-9 2. Water depth increases abruptly—waves don’t break in region—longshore current is disrupted. This can occur at the entrance to a bay. What happens? Human-induced Erosion and Deposition Beaches left to their own devices form a delicate balance betwen material deposited and removed. Our actions can disrupt this process whether intentionally or not. It does not even have to be at the coast: For example: damming a river upstream may cut off a rivers supply of sediment. Without new sediment brought in to beaches what will happen to the beaches? Breakwaters: Groins Sometimes structures are built parallel to the shore to intercept and dissipate wave energy. How? Groins are features built out perpendicular from the shore. How do groins work? With less wave energy hitting the shore what would you expect to happen? What happens downcurrent? Might be good if one is trying to protect a beach. Not so good if one is trying to protect a harbor which now gets clogged up with sand. Not very neighborly!